Hesitations at speed

[Wild Bill's ‘98 TJ]

Hellllllp!
I have a 1998 TJ with 4.0. 150k miles.
I’m the second owner and it is mint condition and all original, except for wheels and tires.
Recently, when traveling a constant 55 or 60mph, it will make a few (very quick) lunges, hesitations, etc. Sometimes when it does it, the rpm gauge will jump quickly from about 27 or 28 to 3500+ (very quickly). Most times, it does the hesitation but the rpm doesn’t change. It also never sets off the check engine light. You know jeeps and people, we start reading articles or forums and get a thousand different opinions. Based on this info, what is your opinion?
O2? TPS? I don’t really want to just throw parts at it.
Thanks so much!

 

[macleanflood]

Read live ODB data. A simple $30 Bluetooth ODB dongle and the free app Torque Lite would be great.

I'd also check and monitor your fuel pressure and clean your battery terminals and grounds.

-Mac

 

[Wild Bill's ‘98 TJ]

Thanks for the reply. The battery and cable ends are relatively new, but I’ll take a look.
Any particular dongle better than others?
Thanks again!

 

[tworley]

Likely you'll get a misfire code soon. Check the voltage to your ignition coil pickup (CAM) sensor and/or crank position sensor

 

[Chris]

I second the OBDII scanner with live data. That will tell you quite a bit and probably help you track the issue down without having to just throw parts at it.

I use one of these on all my vehicles and it works great:
https://www.amazon.com/Vgate-Bluetooth-Scanner-Android-Compatible/dp/B06XGB4873/?tag=wranglerorg-20

 

[topfuel83]

It looks like you have the 32RH automatic transmission. To me it sounds like something is slipping at highway speed causing the RPMs to jump up. I would think a sensor issue would probably cause the RPMs to drop.

You mentioned the wheels and tires were charged. Is the tire diameter larger than factory? If the axle gearing wasn't changed with larger tires it can put more stress on the transmission that might be starting to show up.

 

[Wild Bill's ‘98 TJ]

It looks like you have the 32RH automatic transmission. To me it sounds like something is slipping at highway speed causing the RPMs to jump up. I would think a sensor issue would probably cause the RPMs to drop.

You mentioned the wheels and tires were charged. Is the tire diameter larger than factory? If the axle gearing wasn't changed with larger tires it can put more stress on the transmission that might be starting to show up.

Yes, it had 18” Jk wheels on it when I bought it. I have no idea if the previous owner changed the axle gear. I’ll have to check into that.
**On a side note, several post suggested I unplug the upstream O2 sensor and drive it to see if it still did the occasional “buck” thing.
I attempted that, but my Jeep won’t crank with the O2 sensor unplugged. It would only turn over. I plugged it back up and it cranked.
Thanks for the info.

 

[macleanflood]

That's interesting. That upstream O2 sensor shares the same path as your crank position sensor.

I can run my 97 no issues with both O2s removed.

I'd start unwrapping your harness in that area and look for damage... especially heading around the back of the valve cover.

-Mac

 

[Wild Bill's ‘98 TJ]

I didn’t understand it either. I was 99.7% sure I could unplug the O2 and still run it. I literally unplugged it, tried to crank it, would not crank, plugged it back in, cranked immediately, and drove it about about a 40 mile trip to visit my mom. Equally as strange, it didn’t buck a single time there and back.
It always runs like a well oiled machine and even did the times it was doing the quick buck/lunge/rpm jump.

 

[KennethS]

To me it sounds like something is slipping at highway speed causing the RPMs to jump up. I would think a sensor issue would probably cause the RPMs to drop.

Years ago my TJ with a manual transmission did the same thing, with engine bucking and tach jumping up. Definitely not a transmission slipping issue. Replacing the crank position sensor cured it for a while, but it started again. I put the original crank sensor back in and it ran well. My suspicion is it’s an issue with the wiring harness somewhere and moving the wiring around while replacing the sensor caused the change in operation.